In recent years, using horizontal wells to improve reservoir-encountered rate and recovery ratio of oil and gas reservoirs has been widely applied in various oil fields. A near-bit geology-oriented drilling system has characteristics of determining properties of strata in real time, exploring to-be-drilled strata in advance, implementing accurate orientation, and the like, which helps to improve discovery rate of exploratory wells, reservoir-encountered rate of development wells and recovery ratio of oil and gas fields.
A near-bit wellbore drift angle measurement instrument is usually to package a near-bit wellbore drift angle sensor at the front end of a deflection tool (downhole mud motor), transmit data into a conventional wireless measurement while drilling (LWD/MWD) instrument positioned at the upper part of the deflection tool (downhole mud motor) by means of a wired or wireless transmission (ultrasonic wave, electromagnetic wave, etc.), after that transmit the signals to the surface by means of a wireless transmission manner such as mud pulse telemetry or electromagnetic wave, together with the data measured by the conventional wireless measurement while drilling instrument.
Real-time acquisition of the near-bit well deflection parameter and gamma measurement helps field drilling engineer to control the drilling trajectory of the bit and geologic parameters in drilling process, thereby improving the oil drilling and production rate. However, how to transmit parameters measured near the bit to the ground becomes the key to the development of the technology.
At present, most of domestic researches focus on transmitting the parameters measured near the bit by means of a cable embedded in the mud motor (e.g., a screw-type mud motor), an acoustic wave wireless transmission manner, a coil type electromagnetic wave wireless transmission manner, and the like.
The cable buried in the mud motor belongs to a wired transmission structure: near-bit stratum information measured by a sensor within a measurement sub is transmitted to the MWD system through a wire or cable, and a cable channel is established between the near-bit measurement sub and a receiving sub. However, these “cables” are embedded in each mechanical part between the measurement sub and the MWD system, and a wired channel is established, that is, there are needs for pre-burying cables in downhole drilling tools such as the measurement sub, a receiving sub and a mud motor, for a special screw motor to be suitable for signal transmission, and for solving problems of high-pressure sealing and reliable electrical connection of thread interfacing positions, resulting in low applicability.
Compared with the wired transmission manner, a wireless transmission method is lower in overall cost and better in versatility, and can be used in downhole small-data-volume transmission since there is no need for largely changing a structure of the downhole drilling tool in a manner of upwards transmitting the parameters measured near the bit. However, in an existing wireless transmission technology, there are still the following shortcomings.
The coil type electromagnetic wave wireless transmission is as follows: electromagnetic wave transmission adopts an electromagnetic wave transmitting apparatus and an electromagnetic wave receiving apparatus. That is, coils for transmitting and receiving electromagnetic wave are respectively wound on independent drill collars, and then covered with an insulating material for protection, so that a wireless transmission apparatus is formed. Such a transmission manner is higher in requirements for resistivity values of strata. Furthermore, as a signal is received depending on the coils on the outer wall of the drill collar, when the signal is transmitted to the ground through MWD, it is necessary to solve a data communication problem between the MWD in a nozzle and the receiving coil on the outer wall of the drill collar. That is, a special structure is designed to perform data connection, which reduces the retrievability of the MWD system.
The acoustic wave wireless transmission is as follows: the acoustic wave wireless transmission manner is to respectively install a transmitting transducer and a receiving transducer at a transmitting sub and a receiving sub of the drill collar, and signals are transmitted by means of acoustic characteristics of the transmitting transducer and the receiving transducer. However, a design of the transducers makes a mechanical structure of the near-bit transmitting sub be very complicated, and increases the length of the near-bit transmitting sub. Furthermore, the acoustic wave transmission cannot meet a requirement for an across-mud motor transmission distance (about 10 meters) as well because of the strong noise produced near the bit. Since an acoustic wave receiving apparatus receives the signal by installing a receiving transducer on the outer wall, it is also necessary to solve a communication problem between the structure transducer on the outer wall and the MWD in the nozzle, which reduces the retrievability of the MWD system.